The present invention generally relates to molds and punches, and the fasteners, screwdriver heads and bits made using them, which employ a Torx®-shaped slot.
As background, drawn wire is pulled to a certain thickness or gauge, and a “header” punch is used to form the head and length of the screw. Threads are formed on the length of the screw using a rolling die. A slot may be formed on the screw head, typically using a stamping process. The stamping process may be a two-stage forming process, utilizing “primary” punches and “secondary” punches. The primary punch provides a positioning-point slot for positioning the secondary stamping. The secondary punch provides the secondary stamping, which forms the slot on the head of the fastener, for example. If the slot is properly designed, a driver bit may be inserted into and fit to the slot in a secure and stable manner, so that the driver bit can be used to firmly and securely rotate the fastener.
The shape of the slot of screws can be slotted, crossed, hexagonal, Torx®, etc. The so-called Torx® slot is a star-shaped slot with at least 6 lobes. As background, Torx®-head screws resist cam-out better than Phillips-head or slot-head screws. While Phillips heads were designed to cause the driver to cam-out, to prevent overtightening, Torx® heads were designed to prevent cam-out. Thus, rather than rely on the tool to slip out of the screw head when a predetermined torque level is reached, which risks damage to the driver tip, screw head and/or workpiece, torque-limiting driver designs achieve a desired torque consistently. The Torx® design allows for a higher torque to be exerted than a similarly-sized conventional hex socket head without damaging the head and/or the tool.
Prior art FIG. 1A shows a conventional hex slot 10, while prior art FIG. 1B shows a Torx® slot 15. Circle 12, passing through the six points of contact between the driver and the slot of the screw/tool head, represents the direction of the rotational force being exerted at each of those points. (The clearance between the components is exaggerated for clarity.) Because the plane of contact is not perpendicular to this circle, a radial force is also generated which tends to “burst” the female component and “crush” the male one. If this radial force component is too great for the material to withstand, it will cause the corners to be rounded off one or both components, or will split the sides of the female part. The magnitude of this force is proportional to the cotangent of the angle β between circle 12 and the contact plane. As seen by a comparison of FIGS. 1A and 1B, the angle between the plane of contact between the tool and the fastener head, and the circumferentially-directed force, angle β, is much closer to 90° for a Torx®-type head, and so for a given torque the potentially damaging radial force is much lower.
Despite its advantages, the Torx® slot has various shortcomings. For example, when a star screwdriver is plugged into the Torx® slot, it is more difficult to align the screwdriver with the slot as compared with slots of other shapes. The Torx® slot also requires more locking points, requiring a higher precision than a common crossed-slot or slotted-slot. Additionally, a screwdriver can easily be rocked or shaken, due to insufficient precision, when plugged into a Torx® slot. Problems are also often encountered during stamping of the Torx® slot, typically due to poor design of the stamping die, as further discussed below.
During stamping of the Torx® slot, a stamping die with a poor design can cause errors in the slot that is formed. Some slots have larger openings than desired, while other slots may have curved side walls. In either case, a screwdriver bit may be shaken or loosened when inserted within the screw slot. Accordingly, there is a need for manufacturers to improve the design of stamping dies used to manufacture Torx® slots on the heads of fasteners and tools such as screwdrivers and bits.
U.S. Pat. No. 8,955,417 discloses a rotary drive design for a tool and for a workpiece to be rotationally driven, including a stamping die for making generally Torx®-slotted fastener heads. Screws or drivers have a manufacturing tolerance difference or gap between the torque-receiving surface of the screwdriver (tool) and the torque transmission surface of the screw slot (workpiece). During use of the rotary drive, this gap can cause the tool to be subjected to forces running transverse to the rotational axis; these transverse forces can reduce the driving speed, and can cause the screw to be threaded into the workpiece at an oblique angle to the longitudinal axis of the workpiece.
The basic cylindrical shape of the rotary drive shown in the '417 Patent has a plurality of rounded projections, and rounded recesses arranged between these projections. The outermost surfaces of the projections tangentially form a circle having a maximum radius, while the innermost surfaces of the recesses tangentially form a circle having a minimum radius. Deviating from this cylindrical basic shape, the regions between the projections form conical surfaces, which are not instrumental in the transmission of torque, but act to center and align the tool with the workpiece.
Still referring to the '417 Patent, the male die is used to press a metal blank, deforming the metal blank in a given direction under controlled pressure. In order to reduce scrape generated due to metal overflow in the slot, a flash groove is disposed on the punch. However, the flash groove lowers the precision of the stamped slot, because the die cavity is not filled. With the male die of the '417 Patent, the projection is vertically oriented, while the conical surface is used to guide metal flow. The slope of the conical surface is quite large. During the stamping, the vertical projection leaves no more space for metal to flow, yet the conical surface provides too much space for the overflow, due to the large slope of the conical surface. Accordingly, because the space in the die cavity is not distributed evenly, burrs form at the edge of the projection. This results in the need for a secondary processing in order to remove the burrs. In the conical surface, the die cavity is unable to be refilled. Thus, not only is precision reduced, but the tool and the workpiece may not be smoothly matched to each other.